Lemma A.1.4 (Karamata Characterization Theorem; Theorem 1.4.1 in [1]}) 

Let \(f\in RV_{\gamma }\) . Then there exists a slowly varying function \(L\)  (that is, \(L\in RV_{0}\) ) such that for all \(x\)
\(f(x)= x^{\gamma } L(x).\) 

